1. Field of the Invention
The present invention relates to a head-up display (hereinbelow, referred to as a HUD) for displaying a driving information to a driver in a vehicle, a combiner used for the same and a method of designing the head-up display.
2. Discussion of Background
Recently, there has been used a HUD as a method of displaying information to a driver in a vehicle. The HUD is so constructed that the driver can read information without substantially moving his eye point in his state of driving by displaying an optical information projected from an information projecting means such as a liquid crystal display device or the like on a combiner such as a hologram, a half mirror or the like which is installed at a windshield glass or the like of the vehicle.
In particular, the combiner comprising a hologram can diffract an optical information to the eyesight direction of the driver because the hologram has a feature capable of diffracting an optical information to the driver as well as functioning as a lens. Further, it can form an image at an optional position without using an optical system such as a lens or the like and can display an image having a high luminance without deteriorating the luminance of the foreground. Therefore, the hologram is effective to the combiner for a HUD.
FIG. 11 is a diagram showing an example of a conventional HUD. Light 43 including information to be displayed, emitted from a light source 46 and passed through a transparent type liquid crystal display element 45 through a lens system 44, is irradiated on a hologram 42 arranged in a windshield glass 47 of a vehicle body. The light is diffracted at the hologram to be seen by the driver at his observing position 41.
The lens system 44 functions as a collimator. Further, the function of the lens can be shared by the hologram 42, and a speed displaying image 48 and an alarm displaying image 49 can be formed at a distant place.
Since the hologram 42 has a specified wavelength selective function, it is possible to display an image having a desired color. Usually, color to be displayed is single. However, a display having plural kinds of color is possible by means of multiple exposure to light so that the quantity and the quality of information to be displayed can be improved. For instance, the speed displaying image 48 is shown in a green color and the alarm display image 49 is shown in red whereby it is possible to transmit correctly information to the driver.
As described above, since the conventional HUD diffracts the light having a specified wavelength, when the driver looks at something outside through the hologram, the color of transmitting light is the complementary color of a displayed color corresponding to the diffraction wavelength of the hologram. Namely, as shown in FIG. 8, when a displayed color is only green, a white light 72 from the outside is partly reflectively diffracted by a diffraction grating in the hologram during the transmission of the light through the hologram 71, and the reflection light 73 becomes green. Accordingly, the transmission color of a transmitting light 74 is magenta (pink-red) as the complementary color of the diffraction light, and the driver 75 may feel stimulative and uncomfortable to the transmission color. Further, the color tone of the background is also influenced by the complementary color, and visibility to circumstances such as a road or the like outside of the vehicle is impaired during cruising whereby there is a problem of safety.
On the other hand, as shown in FIG. 9, with respect to an observer 85 outside of the vehicle (a walking passenger or a driver in an opposing car), the color of a reflection light of outer light 82 is substantially changed from a red color 83 (observation from the front) to a green color 84 (observation from a slant position) depending on an angle of observation, due to the angle dependence of a diffraction wavelength by the diffraction grating in the hologram 81. The color change also gives uncomfortable impression. Therefore, there were problems in the design and the quality of the combiner used, hence the vehicle itself which installs therein the combiner.
FIG. 10 is a chromaticity diagram showing a result of simulation concerning the color change of reflection colors. The chromaticity diagram is to show quantitatively colors by chromaticity coordinates x and y ruled in JIS-Z8701 (Japanese Industrial Standards, which provides specification of colors according to the CIE (Commission International de I'Eclairage). In FIG. 10, a point indicated by a mark x as a light source expresses a white color. When the color of the hologram is closer to this point, the color is closer to a white color, which is more preferable to an observer.
However, the color of a reflection light of a monochromatic hologram used for the conventional HUD is changed as shown by marks .diamond-solid. in FIG. 10 when the hologram has a wavelength of 545 nm and a diffraction efficiency of 60%, for instance. Namely, it is understood that a red color is provided around a front position and there is a great color-change from an orange color through an yellow color to a green color as the position of observation shifts to a further oblique position. When the hologram is viewed from the front, a red color as a stimulative color is provided, and when it is viewed from a slant position, it looks a glaring green color having a high luminous efficacy is provided to thereby give an uncomfortable impression to the observer.
In order to improve the color tone of a reflection light to an observer outside a vehicle and in order to improve the color tone of a reflection light by a first hologram used for display, Japanese Unexamined Patent Publication No. 110984/1992 and U.S. Pat. No. 5,153,751 disclose a HUD wherein a second hologram for reflecting the color near the complementary color of the reflection color of the first hologram is laminated or exposed multiply to light. However, since the proposed hologram was formed with two holograms corresponding to two colors, there was sometimes impossible to improve completely the color tone. In particular, there was no light having the wavelength corresponding to the complementary color with respect to a green color light (having a wavelength of about 500-560 nm) which is frequently used for a display color. Accordingly, the proposed technique had a drawback which was difficult to improve the color tone of the reflection light and could not improve the color tone of a transmission light.
Further, in order to improve the color tone of a transmission light through the combiner, Japanese Unexamined Patent Publication No. 179418/1991 discloses a HUD in which an optical element capable of reflecting or attenuating light having a wavelength .lambda..sub.2 which is a complementary color of a reflection wavelength .lambda..sub.1 of a hologram for displaying is combined. In this publication, however, since color correction was made by a two color structure in the same manner as the above, there was a similar drawback that complete improvement could not be achieved with respect to a green color light, and an improvement in the color tone of reflection light was not made.
Japanese Unexamined Patent Publication No. 291221/1992 discloses a HUD in combination of an optical element which reflects or attenuates light having a plurality of wavelength regions capable of synthesizing light having a wavelength region which forms the complementary color of a reflection wavelength .lambda..sub.1 of a hologram for displaying, in order to improve the color tone of a transmission light of a combiner to a green color light. In this publication, since improvement to only the color tone of transmission light is made, it is effective only to a specified angle connecting the hologram to an observer. Accordingly, the improvement of the color tone of the reflection light by the hologram to an observer outside of the vehicle was not always made. For instance, in an example of the publication, a hologram for diffracting light of 460 nm and 630 nm which is capable of synthesizing the complementary color of light of 530 nm is used in order to improve the color tone of the transmission light of the hologram which diffracts light of 530 nm. Further, a transmission characteristic diagram wherein there are a diffraction efficiency of about 90% and a halfwidth of about 10 nm is shown. Simulation of the color tone of a reflection color of the combiner based on the characteristic diagram in a case that the reflection color is observed outside of the vehicle results in as shown in FIG. 12. From the Figure, it is understood that the color is a yellowish green in observation from the front although it is closer to a white color in observation from an oblique position.
As described above, although proposals were made to use a plurality of holograms in order to improve the color tones of reflection lights and transmission lights of a holographic combiner, there were problems that a sufficient effect of improvement has not been obtained since the characteristics could not be optimized.